1. Field of the Invention
The present invention relates to a block-based moving picture encoding method, and more particularly, to a method which calculates a motion vector based on a transformed picture obtained by Discrete Coefficient Transform (DCT) or a Wavelet transform, and an apparatus appropriate for the method. The present application is based on Korean Patent Application No. 2001-66745, filed Oct. 29, 2001, which is incorporated herein by reference.
2. Description of the Related Art
Roughly speaking, three methods are used for compression of digital image data. The three methods include a method which reduces temporal redundancy, a method which reduces spatial redundancy, and a method which reduces data using the statistical characteristic of a generation code. A representative method for reducing temporal redundancy is a motion estimation and compensation method.
FIG. 1 is a block diagram of a prior art motion estimator. In FIG. 1, an input frame image is divided into Discrete Cosine Transform (DCT) blocks, each of which has 8×8 picture elements. In a DCT transform process S102, input DCT blocks are DCT transformed and output. As the result of the DCT transform process S102, each DCT block has 8×8 DCT coefficients.
In a quantization process S104, DCT coefficients are quantized according to the quantization step size, and output. The output of the quantization process S104 is provided to a variable length encoding process (not shown). In the variable length encoding process, statistical coding for the quantized DCT coefficients are performed in units of a macro block. Also, statistical coded image data, a motion vector provided by the motion estimation process S108, and additional information such as the type of a macro block are multiplexed and output. Here, the macro block is usually formed with 6 DCT blocks, including 4 blocks of a luminance component and the remaining 2 blocks of a color difference component. The motion estimation is performed on the blocks having the luminance component.
Meanwhile, for motion estimation, the DCT block, which is output in the quantization process S104 and has quantized DCT coefficients, is reproduced as a DCT block having 8×8 picture elements again through the inverse quantization process S106 and the inverse-DCT transform process S108.
In a motion estimation process S110, a motion vector for each macro block is estimated with respect to a reproduced frame image ((K−1)-th frame) and an a frame image being input (K-th frame).
The motion vector estimated by the motion estimation process S110 is provided to the motion compensation process (not shown).
As shown in FIG. 2, if the maximum displacement is P and the size of a block is N×N, for motion estimation, a search area 206 of W (=N×2P)×W (=N×2P) in a (k−1)-th frame is determined, and a block 204 which has the optimal similarity to the N×N block 202 in a k-th frame is searched for and obtained in the search area of the (k−1)-th frame. By calculating the displacement, a motion vector (MV) is obtained. As evaluation functions for measuring similarity, Mean Square Error (MSE), Mean of the Absolute Differences (MAD), and Sum of the Absolute Differences (SAD) are widely used.
Since the prior art motion estimation method as shown in FIG. 1 performs motion estimation on an original picture having picture element data, inverse quantization process and inverse-DCT transform process are needed. As a result, the process for estimating motion is delayed, which lowers the processing speed of moving picture encoder.